|You really didn't provide enough info about your computer but it appears it only has 512MB RAM? 512MB really isn't enough memory to run XP decently, so if you install Vista, the system will be even slower. In other words, do NOT install Vista! If possible, increase your memory amount to at least 1GB & stick with XP.|
As the performance level of gaming video cards have increased, their power requirements have grown beyond what the AGP slot can supply - these are the cards that require an additional power connection from the power supply. The Geforce 6200 is an old, slow, low end video card that is NOT good for playing games. And since it is such a low end card, it doesn't require an added power connection.
Just because a card has 256MB doesn't mean it's fast. There are plenty of cards with just 128MB memory that are much faster than some 256MB cards. You have to look at ALL the specs - graphics chip model, core speed, memory type (DDR, DDR2, DDR3, etc), memory buswidth (32-bit, 64-bit, 128-bit, etc), pixel pipelines, shader support, DirectX support, etc.
Here's a tip when shopping for older nvidia cards in the 5000 thru 9000 series. The 1st digit is the generation (5 thru 9), so obviously the 6200 is fairly old. The 2nd digit is the performance level (1 thru 8). Generally speaking, gaming cards will have a 6 or higher, so the 6200 is not a gaming card. There are also "tag letters" at the end of the model number (LE, SE, XT, GS, GT, GTS, etc.) that further define the performance level within a series. LE, SE & XT are lower performance, GS, GT, GTS are higher performance. Cards with no tag letters are called "plain vanilla". Using the Geforce 6600 series as an example, the 6600SE & 6600LE have lower performance than the plain vanilla 6600. 6600GS & 6600GT have higher performance than the plain 6600.
ATI cards use a different numbering system & different tag letters. DX9 support started with the Radeon 9500 so I don't recommend getting a card that's numerically lower than that. Another difference is the tag letters, especially the use of XT. For ATI cards, XT is higher performance than the plain vanilla. Using the Radeon 9600 series as an example, here's the list from lower to higher performance - 9600SE, 9600, 9600Pro, 9600XT. A "best buy" in the early Radeon series was the 9550 with 128-bit memory. It was basically an underclocked 9600, but usually sold for considerably less money. It could be easily overclocked to 9600 level performance using a program called ATiTool.
The following charts should be helpful: